The present invention relates in general to sewing machines and in particular to a new and useful device for moving the main shaft of a sewing machine to a predetermined position after the sewing machine has come to a stop.
In some instances, after stopping a sewing machine in a certain position, for example with the needle in its uppermost or lowermost position, it is desired to turn the main shaft back, through a certain angle, into another position. For example, if in a chain stitch sewing machine, the rotary hook or looper has engaged the needle thread loop and the needle is in its uppermost position, the looper must be disengaged to be able to remove the work without hindrance.
In lock stitch sewing machines, the take-up lever in its upper return zone lags the needle bar by about 30.degree. of rotation of the main shaft, i.e. the needle bar, after having passed its uppermost position, already moves downwardly while the take-up lever still moves upwardly. Because of this fact, which is required by the sewing technique, and since the thread is neither clamped nor otherwise retained after the sewing machine has been stopped and after the thread has been cut, with the needle in its uppermost position, upon restarting the sewing machine, the thread is pulled out of the needle eye by the upwardly moving take-up lever, and restarting of the sewing operation may be delayed by the necessity of threading the needle again. Therefore, frequently, the uppermost position of the take-up lever is chosen as the stopping position after the thread cutting operation. Since, however, at this point the needle bar is already moved a certain distance downwardly, in many instances the clear space below the needle becomes insufficent for an unhindered removal or insertion of the work. The operator must use the handwheel and turn the machine back through a certain angle into the upper position of the needle. This is a time-consuming complication.
German Pat. No. 910,023 discloses a sewing machine drive, where for stopping the machine in predetermined positions, the main shaft of the sewing machine first is brought from a full speed rotation to a lower speed at which it rotates up to a predetermined position and stops, whereupon it is automatically turned back through a certain angle and again stopped. The speed and reversal are effected directly by the drive motor which is equipped with an electron tube control. Such a drive is relatively complicated and the electronic control is rather susceptible to disturbances.
Another drive disclosed in German Pat. No. 1,438,338 U.S. Pat. No. 3,170,425 comprises a coupling motor with a permanently rotating main drive disc and an axially displaceable coupling disc which is operatively connected to the sewing machine and, after being disengaged from the main disc, can be brought into operative connection with an auxiliary drive, to be further rotated into a predetermined position in which the machine is stopped.
To rotate the sewing machine shaft automatically back through a certain angle and stop it again, a second auxiliary drive is provided at the shaft of the first auxiliary drive, which is equipped with an electromagnetic clutch and a brake and rotates oppositely to the first auxiliary drive. The operation of the clutches and brakes of the two auxiliary drives is such that to stop the sewing machine for the first time, the clutch of the first auxiliary drive is disengaged and simultaneously the brakes of both the auxiliary drives are engaged. At that point the clutch of the second auxiliary drive is engaged while simultaneously disengaging the brakes of both of the auxiliary drives, and is disengaged again, with a simultaneous engagement of the brakes of both of the auxiliary drives, as soon as the predetermined return position of the shaft of the sewing machine is reached.
The clutches of the two auxiliary drives may be operatively connected through belt drives to the coupling motor or to auxiliary motors.
The stop after return is effected through an additional sliding contact ring with a point of interruption, of a synchronizer which is usually provided in drives of this kind. This drive arrangement works satisfactorily, however, the second auxiliary drive and its control make the construction rather expensive.